1. Field of the Invention
The present invention relates to a shift-by-wire system, which changes a shift range of an automatic transmission of a vehicle.
2. Description of Related Art
In recent years, in the field of vehicle control technique, by-wire systems have increasingly been employed. In general, the by-wire system electrically controls a corresponding actuator, which changes a corresponding operational state of the vehicle, based on a command of an occupant of the vehicle. For example, a shift-by-wire system, which changes a shift range of an automatic transmission of the vehicle based on a command of the occupant of the vehicle, is known (see, for example, Japanese Unexamined Patent Publication No. 2001-271917 and Japanese Unexamined Patent Publication No. 2004-513307 corresponding to U.S. Pat. No. 6,752,036).
It is required to enable secure parking of the vehicle and towing of the vehicle at the time of encountering an abnormality in the vehicle. In the case of the preexisting vehicle, the vehicle can be parked by changing the shift range of the automatic transmission to the P range or by using a parking brake. Furthermore, the vehicle can be easily towed by changing the shift range of the automatic transmission from the P range to another range.
In the case of the vehicle, which has the shift-by-wire system, the shift range of the automatic transmission is electrically changed. Therefore, for example, when an abnormality occurs in an electric power source or an electric power supply system (e.g., battery exhaustion, breaking of a conductive line), it becomes difficult to change the shift range of the automatic transmission. Furthermore, in the recent years, the parking brake is also electrically actuated. Thus, when the abnormality occurs in the electric power source or the electric power supply system, the secure parking of the vehicle may not be possible. Furthermore, when the shift range is changed to the P range, releasing of the P range may possibly become difficult, and thereby towing of the vehicle may also become difficult.